powerpc/numa: use cached value of update->cpu in update_cpu_topology

There isn't any need to keep referring to update->cpu, as we've already
checked cpu == update->cpu at this point.

Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>